Stop, in music, on the organ, mechanism controlling the entry of air from the pressurized wind chest into a rank of pipes producing a distinctive tone colour. The word stop also denotes, by extension, the register, or rank of pipes, controlled by a stop. Stop also occasionally refers to mechanisms altering the tone colour of the strings of harpsichords and early pianos.

Ranks of organ pipes controlled by stops.

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The earliest organ stops used a slider system. Holes in a strip of wood set in a slider frame coincided with holes in the feet of the pipes of one register. By pushing a knob the organist could slide the holes slightly beyond the pipe feet, blocking entry of air into those pipes. An alternate method was introduced in the 20th century, with electrically operated valves controlling entry of air into the pipes.

Each rank of pipes, such as the diapason, is controlled by a separate stop. Mutation stops consist of pipes sounding higher (e.g., by five notes) than the other pipes, rather than in unison with them. Used in combination with unison pipes they add an incisive quality to the sound. Mixture stops consist of two or more ranks of pipes, both unison and mutation ranks, controlled by a single stop.

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in acoustics, sound that can be recognized by its regularity of vibration. A simple tone has only one frequency, although its intensity may vary. A complex tone consists of two or more simple tones, called overtones. The tone of lowest frequency is called the fundamental; the others, overtones. The...

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It has already been explained that the pitch of any pipe is proportional to its length. Most modern organs have a manual compass of five octaves, from the second C below middle C to the third C above; an open pipe sounding the low C is about 8 feet (2.5 metres) in speaking length (64 vibrations per second). The shortest pipe in the same rank, or stop, is thus about 3 inches (8 centimetres) long...

...to all the pipes belonging to that key; but, in order that the organist may be able to use any of the ranks of pipes, alone or in combination, an intermediate mechanism is provided by which he may stop off any rank or ranks. From this function the control by whose operation the ranks are stopped off has come to be known in English as a stop, a term also used loosely for each rank of pipes.